Impedance cardiography is a non-invasive technique for determining cardiac performance in humans. When such equipment is employed, a high frequency electric signal is applied to the patient across outer electrodes positioned, for example, on the patient's head and lower thorax. Voltage differences between sensing inner electrodes positioned between the outer electrodes on the patient's neck and chest are measured and used to compute an impedance (Z). The impedance is based on the low magnitude, known electrical current passing between the outer electrodes.
In 1932, Atzler and Leyman reported that cardiac output of a human could be determined by such impedance methods in Uber ein neues Verfahren zur Darstellung der Herztatigkeit (Dielektrographie), Arbeitsphysologie, 5:636-680. In 1966, Kubichek reported the ability to correlate changes in base line impedance and the first derivative of impedance to stroke volume (SV) according to the following Equation (1) disclosed in U.S. Pat. No. 3,340,867 and in the publication: Development and Evaluation of an Impedance Cardiac Output System, Aerospace Medicine, 37: 1208-1212. ##EQU1## where: p1 .rho. is the resistivity of blood; L is the spacing between the sensing electrodes;
Z.sub.0 is an average or baseline impedance; and ##EQU2## is the magnitude of the peak negative value of the time derivative of the impedance Z for a period of time, typically a second.
Cardiac output may be deduced from stroke volume by multiplying the latter times the heart rate.
Although the Kubichek formula provides a value that correlated with cardiac output, the absolute accuracy of the method remained doubtful and, in particular, subjects with certain cardiovascular problems show values with great inaccuracies.
In 1982, Sramek proposed a modification of the Kubichek formula of equation (1) which resolved the base line impedance Z.sub.0.sup.2 into a dynamic and static component as reported in the publication: Cardiac Output by Electrical Impedance, Med. Elect., 2:274-290.The static term Z.sub.0 s was described by the following equation: ##EQU3## where A is the area of the thorax being measured.
The dynamic component Z.sub.0 d was simply the baseline or average of the impedance being measured: EQU Z.sub.0 d=Z.sub.0 ( 3)
Incorporating the static term and dynamic term into the Kubichek equation provides the following formula: ##EQU4##
The value of A may be estimated by approximating the chest as a cylinder in which case equation (4) becomes: ##EQU5## where C is circumference of the chest near the area of measurement. Alternatively, Sramek proposed that the term ##EQU6## be replaced with either ##EQU7## where H is the height of the patient because 1.7*H approximates L.
These approximations did not produce good results and so Sramek was ultimately led to produce a set of charts attempting to establish correlation between area and the three factors of gender, height and weight.
In 1986, Bernstein proposed a modified equation in which the separation of the electrodes and the height of the patient were considered, in the following form: ##EQU8##
All of the above methods suffer from lack of accuracy and indicate, in some subjects, falsely high or low values of stroke volume.